Abstract

In this review I will provide insight into the existence of the gut-microbiota-brain axis, connecting the gut, the microbiota, and the brain with one another and focus on microbiota’s prevalence in autism spectrum disorders. I will try to clarify the possible underlying mechanism resulting in autism spectrum disorders in a microbial dependent and independent way.
Humans have been a host for microorganisms for thousands of years in which both the host and the microorganisms benefit. This is called a mutualistic symbiosis which is shaped due to co-evolution and has been of major importance ever since. Disruption of this delicate interplay between the host and the microorganisms in its gut, called microbiota, may result in inflammatory bowel diseases or even neurologic disorders. Autism spectrum disorders generally known as a neurodevelopmental disorder, have been associated lately with this microbial imbalance. Microbial imbalance, dysbiosis, was known to result in intestinal diseases like inflammatory bowel disease but not in neurologic disorders. This interesting finding resulted in the hypothesis in which they described a mechanism connecting the gut with the brain and vice versa leading to this unexpected outcome. They called it the gut-microbiota-brain axis.
A study in patients, focussing on the effect of microbiota on phenotypical ASD characteristics, demonstrated microbial influence leading to a decrease in ASD characteristics when using antibiotics. This observation confirmed the gut-microbiota-brain axis hypothesis and its effect on ASDs. Follow-up studies showed differences between the microbiota of healthy controls and patients with ASDs. This study displayed a possible microbial origin of ASDs and led to the finding that the gut-microbiota-brain axis was influenced in a neuro- and enterotoxin dependent manner. This finding was not completely out of the blue as both the enteric nervous system and the brain use the same neurotransmitters and signalling molecules. However underlying mechanisms of how the gut-microbiota-brain axis functions remains unclear.
I furthermore tried to elicit the underlying origins resulting in ASDs via the gut-microbiota-brain axis. But the brain in general and the origin of ASDs are both not completely understood. Fully understanding ASDs on itself is therefore very difficult and connecting the origin of ASDs to the gut-microbiota-brain axis is even more difficult.
I will give a brief explanation of several different origins resulting in ASDs in a microbial dependent and independent way.